The Ackerman steering principle holds that correct (i.e., perfect or ideal) steering requires the projected axes of a vehicle's steerable front wheels to intersect each other on a projection of the rear axle centerline during any turn so that all wheels will have pure rolling motion about the common point of intersection. A general appreciation of the Ackerman steering principle within the automotive industry has led to the near-universal adoption of steering mechanisms having geometries designed to approximate correct Ackerman steering. A fundamental requirement of Ackerman steering is that, during a turn, the inside steerable wheel must have a greater steer angle, relative to the vehicle's longitudinal centerline, than the outside steerable wheel; the difference in steer angles between the steerable wheels being termed the toe angle. The toe angle necessary for correct Ackerman steering is a function of the vehicle's turning angle (i.e., the angle between the vehicle's centerline and its direction of travel), its track width and wheelbase. Therefore, the typical vehicle steering system has components, including steering arms, tie rods, idler arms, adjuster sleeves, ball joints, center links, steering knuckles and king pins (collectively referred to in the industry and herein as steering linkage), designed such that the toe angle between the steerable wheels varies to approximate correct Ackerman geometry as the steerable wheels are moved.
In practice, however, a typical steering linkage actually produces perfect Ackerman steering for only one turning angle of the vehicle. For all other turning angles, the toe angle produced by the steering linkage produces some Ackerman error (i.e., a mis-match between the point of intersection for the axes of steerable wheels and the rear axle line), also known as steering error. This steering error creates slippage problems because the steerable wheels and other wheels of the vehicle are not moving around a common radius point. This slippage can result in increased wear on the tires and reduced traction for the vehicle.
Many approaches for reducing the Ackerman steering error problem have been suggested. The majority of such approaches take the form of methodologies for selecting the geometric parameters for a conventional steering linkage, i.e., a steering linkage in which each constituent component has a fixed configuration. For example, a research paper entitled "Use of Computers in Steering Geometry Analysis" by R. B. Kazmier, Section #6 of SAE Paper SP-240 presented December 1962 describes analytical, semi-graphic, and computer-based methods for the calculation of geometric parameters for conventional steering linkages to reduce the Ackerman steering error problem. While such approaches are useful in reducing the Ackerman error in conventional steering linkages, it is nonetheless generally recognized that conventional steering linkages can totally eliminate the Ackerman steering error at only one turning angle. Therefore, some degree of Ackerman steering error remains for the majority of turning angles.
When conventional steering linkages are used, any change in the steering-related parameters (e.g., wheelbase, wheel track, axle weight rating) between different vehicle models will necessitate the calculation of new steering geometry in order to minimize the Ackerman error. Even a change in load on the vehicle will change the steering geometry. Further, steering linkage components having different configurations must be kept in inventory to accommodate each different geometry. Thus, when a manufacturer offers a large number of models, it can become very expensive to provide steering linkage which is optimally tailored to minimize Ackerman error for each model.
With conventional steering linkages, the configuration and physical location of the linkage components with respect to the other vehicle components, such as the chassis, is largely dictated by the required geometry. In some cases, the linkage geometry needed to approximate correct Ackerman steering interferes with the positioning of other equipment on the vehicle. For example, on some long wheelbase trucks, the configuration of the tie rod arms needed to approximate correct Ackerman steering would cause the tie rod arms to interfere with certain popular wheel rims. As a result, correct Ackerman geometry is normally sacrificed to allow the customer their choice of wheel rims.
To overcome the limitations of conventional steering linkages, another approach to addressing Ackerman steering error involves the use of active steering angle adjustment systems in which one or more steering linkage components have a configuration which changes automatically to change the steering geometry as the vehicle is being operated. One such approach is described in U.S. Pat. No. 5,143,400 which discloses a system that uses a computer to actively adjust the toe angle between a steerable slave wheel and steerable control wheel based upon measurements of the actual steer angle of the slave wheel and the calculated steer angle of the control wheel. The system disclosed in U.S. Pat. No. 5,143,400 uses a mechanical actuator to continuously adjust the length of the steering assembly tie rod, thereby adjusting the toe angle between the steerable wheels.
The use of active steering angle adjustment systems can overcome some of the drawbacks found in fixed length steering linkage systems. However, several limitations remain in the active steering angle adjustment systems disclosed in the prior art. Most importantly, a steering angle adjustment system which measures the steer angle from only one of the steerable wheels is subject to error because all of the toe angle corrections generated by the system must be based on a calculated steer angle for the second steerable wheel. The calculations are typically based on the theoretical (i.e., "as designed") geometry of the steering linkage rather than the actual (i.e., "as built") geometry which exists after the linkage is installed in a vehicle. Differences between the theoretical and actual geometries introduce errors into the steering angle calculations. In addition, such calculations can also be rendered inaccurate by a variety of other factors, for example if the steering linkage becomes worn, bent or is modified, or if the load on the vehicle changes. At a minimum, the new parameters of the vehicle must be re-entered into the steering system computer after any such change to maintain steering accuracy. A need therefore exists for a steering angle adjustment system in which steering accuracy can be determined independent from any knowledge of the steering linkage geometry.
Another limitation encountered in prior art active steering adjustment systems is that these systems are not designed to fail in a safe mode. In the event of a failure in the active steering angle adjustment system, it is critical that the toe angle between the wheels not be allowed to drift, change uncontrollably, or become locked in a position that could adversely affect control of the vehicle. A need therefore exists for a steering angle adjustment system which would return the toe angles between the steerable wheels to a predetermined geometry in the event of a failure in the system.